1. Field of the Invention
This invention relates to a surface plasmon sensor for quantitatively analyzing a material in a sample utilizing generation of surface plasmon, and more particularly to a surface plasmon sensor in which the light detecting system is simplified in structure.
2. Description of the Related Art
In metal, free electrons vibrate in a group to generate compression waves called plasma waves. The compression waves generated in a metal surface are quantized into surface plasmon.
There have been proposed various surface plasmon sensors for quantitatively analyzing a material in a sample utilizing a phenomenon that such surface plasmon is excited by light waves. Among those, one employing a system called "Kretschmann configuration" is best known. See, for instance, Japanese Unexamined Patent Publication No. 6(1994)-167443.
The plasmon sensor using the Kretschmann configuration basically comprises a prism, a metal film which is formed on one face of the prism and is brought into contact with a sample, a light source emitting a light beam, an optical system which causes the light beam to enter the prism so that various angles of incidence of the light beam to the interface between the prism and the metal film can be obtained, and a photodetector means which is able to detect the intensity of the light beam reflected in total reflection from the interface for the various angles of incidence.
In order to obtain various angles of incidence of the light beam to the interface, a relatively thin incident light beam may be caused to impinge upon the interface while deflecting the incident light beam or a relatively thick incident light beam may be caused to converge on the interface so that components of the incident light beam impinge upon the interface at various angles. In the former case, the light beam which is reflected from the interface at an angle which varies as the incident light beam is deflected may be detected by a photodetector which is moved in synchronization with deflection of the incident light beam or by an area sensor extending in the direction in which reflected light beam is moved as a result of deflection. In the latter case, components of light reflected from the interface at various angles may be detected by an area sensor.
In such a plasmon sensor, when a light beam impinges upon the metal film at a particular angle of incidence .theta.sp which is at least as great as the angle of total internal reflection, evanescent waves having an electric field distribution are generated in the sample in contact with the metal film and surface plasmon is excited in the interface between the metal film and the sample. When the wave vector of the evanescent waves is equal to the wave number of the surface plasmon and wave number matching is established, the evanescent waves and the surface plasmon resonate and light energy is transferred to the surface plasmon, whereby the intensity of light reflected in total reflection from the interface between the prism and the metal film sharply drops.
When the wave number of the surface plasmon can be known from the angle of incidence .theta.sp at which the phenomenon takes place, the dielectric constant of the sample can be obtained. That is, ##EQU1## wherein Ksp represents the wave number of the surface plasmon, .omega. represents the angular frequency of the surface plasmon, c represents the speed of light in a vacuum, and .epsilon.m and .epsilon.s respectively represent the dielectric constants of the metal and the sample.
When the dielectric constant .epsilon.s of the sample is known, the concentration of a specific material in the sample can be determined on the basis of a predetermined calibration curve. Accordingly, a specific component in the sample can be quantitatively analyzed by detecting the angle of incidence .theta.sp at which the intensity of light reflected in total reflection from the interface between the prism and the metal film sharply drops.
In such a surface plasmon sensor, there has been a problem that since a photodetector which is moved in synchronization with deflection of the incident light beam or a CCD area sensor having a wide light receiving surface is required in order to detect the intensity of the light beam reflected from the interface between the prism and the metal film, the cost of the system is high.